It is known to separate one or more components of a gaseous mixture by using a membrane which is more permeable to the one or more components of the mixture. The gaseous mixtures are brought into contact with one side of the membrane with a pressure differential being maintained across the membrane. Thus, a permeable gas will permeate the membrane to the other side thereof, and, thereby, becomes separated from the gaseous mixture.
In the present instance, oxygen or other rapidly permeating gases, such as carbon dioxide, are separated from other slower permeating gases to produce a gas stream enriched in the rapidly permeating gas. The flow rate of the permeate enriched in the rapidly permeating gas (the desired product) depends upon the permeation rate of the rapidly permeating gas. In the membranes which have low permeation rates, the product flow rate (or productivity) will be correspondingly low. It is an object of the present invention to provide a method for increasing the driving force for permeation of the rapidly permeating gas species and thereby increase its permeation rate and hence the productivity.
Several attempts have been made to provide sweep gas to the permeate side of the membrane so as to allow more of the water and, hence, oxygen to permeate the membrane. For instance, in U.S. Pat. No. 3,536,611, a membrane device is disclosed which uses a sweep stream to sweep a permeated fluid from a bundle of hollow fiber membranes through which the fluid is permeated. The sweep streams are brought in from opposite ends of the bundle of fibers, and it is obvious that exterior piping valves, etc. are required for directing the sweep streams into the device. Such piping and valves are costly and provide opportunities for the gas streams to go awry.
In Japanese Application No. 62-74433, feed gas is added by exterior sources to the feed gas side of the membrane either as a co-current flow or by utilizing external piping and valves. Such piping and valving adds to the cost and complexity of the system.
In U.S. Pat. No. 4,687,578, a major portion of the fiber length is coated, such that the water does not speedily penetrate the fibers, and the remaining fiber length at the non-permeate end of the module is not coated or is only partially coated, wherein the gas permeates rapidly, and, therefore, provides a sweep gas in the module. However, this method is only suitable for using the non-permeate gas to provide a countercurrent sweep over the length of the module.
In U.S. Pat. No. 4,783,201, the membrane was not coated, and in an attempt to allow the water to pass rapidly through the membranes, controlled porosity of the membranes was effected. Here again, a procedure was required to treat the hollow fiber membrane material by a means other than that normally used in production lines. Again, this method is only suitable for using the non-permeate gas to provide a countercurrent sweep over the length of the module.
In the ordinary production of hollow fiber membranes for gas separation modules, the fiber is generally post-treated with a coating material, as defined in U.S. Pat. No. 4,230,463, to correct the defects in the membrane in the form of larger pores which would otherwise allow gas to pass through without the selectivity of the membrane separation. Such membranes which have been treated for defect repair may have low oxygen permeation rates and hence, produce low product flow in oxygen-enriched gas processes, because no method of satisfactorily providing the sweep gas had been provided. The previous methods were inadequate and uneconomical or unsuitable.
The present invention provides a hollow fiber membrane module which provides adequate sweep gas in an economical and easily-produced method.